High resolution x ray diffraction study of phase and domain structures and thermally induced phase transformations in PZN (4 5 9)%PT 3

Figure 5.10 002 RSM taken from the as-polished surface of annealed PZN-4.5%PT, showing the smeared contour lines over the area in the lower 2θ sides of the main 002 R peak despite after

Figure 5.10 (002) RSM taken from the as-polished surface of annealed

PZN-4.5%PT, showing the smeared contour lines over the area in

the lower 2θ sides of the main (002) R peak despite after annealing

at 600 °C for 5 h The intensity contours are on log scale

(a) PZN-7%PT

As-polished surface

(b) PZN-7%PT Fractured surface

(e) PZN-9%PT

As-polished surface

(f) PZN-9%PT Fractured surface

(c) PZN-8%PT

As-polished surface

(d) PZN-8%PT Fractured surface

Figure 5.11 (a), (c) and (e) (002) RSMs taken from the as-polished surface of

PZN-7%PT, PZN-8%PT and PZN-9%PT after annealing at 257

°C for 1 h, respectively (b), (d) and (f) same as (a), (c) and (e) but taken from the fractured surface The intensity contours are

The smeared contour lines over an area of the ω-2θ plane at the lower 2θ side of the

main (002)R peak in the (002) RSM (Figure 5.10) indicates that the diffraction effects

associated with the polishing processes down to 1 µm remain despite annealing to high

temperature

The HR-XRD (002) RSMs were also performed on polished-and-annealed (at

257 °C for 1 h) PZN-7%PT, PZN-8%PT, and PZN-9%PT single crystals The results

are provided in Figures 5.11(a), (c), and (e) To expose the bulk material, the same

samples were later fractured into two halves and were again x-rayed The results are

shown in Figures 5.11(b), (d), and (f)

The polished-and-annealed bulk PZN-PT single crystals were found to cover

with a deformed layer even after heating to above T C, giving rise to smear effect in the

x-ray diffraction patterns (Figures 5.11a, c, and e) As a result, the polished surface can

hardly reveal any useful structural information of the crystals hence unsuitable for

x-ray diffraction studies

It should be emphasized that distinct diffraction peaks were obtained from the

fractured surface (Figures 5.11b, d, and f), suggesting that the fractured surface are

relatively strain-free bulk structure which are more suitable for x-ray diffraction study

5.3.2 Electrical resistance

The effect of room-temperature poling on the stability of the deformed

surface layer was also investigated Figure 5.12 shows the XRD profiles when a dc

E-field of increasing strength was applied to the as-polished sample This figure shows

that the lower 2θ peak is no longer present after the poling field was increased to 1.5

kV/mm Thus, an appropriate poling can eliminate most of the deformed surface layer

However, such a poling treatment should be carried out with care to avoid a non-R

phase or overpoling phenomenon as a result of E-field induced phase transformation in

relaxor single crystals [1, 16, 90, 91]

A different result was obtained when poling was applied on annealed samples

instead As shown in Figure 5.13, the lower 2θ peak persisted in the annealed samples

even after poling to high fields (up to 1.5 kV/mm) The annealing treatment thus helps

stabilize the deformed surface layer such that it becomes more resistant to the poling

treatment

5.4 Summary of main observations

(a) The cause for the occurrence of the extremely broad lower 2θ peak adjacent to

the main (002)R peak in standard XRD profiles of (001)-orientated PZN-PT

single crystals has been examined This diffraction arises from a surface layer

produced by mechanical polishing during the sample preparation stage

Figure 5.12 Effect of increasing poling field on the lower 2θ peak for

as-polished samples without any prior annealing Note that the

lower 2θ peak is largely eliminated after poling to 1.5 kV/mm

Sample thickness is 1mm

2θ

(a) 0.2 kV/mm

(b) 0.7 kV/mm

(c) 1.5 kV/mm

Figure 5.13 Same as Figure 5.12 but for sample annealed at 600 °C for 1 h

prior poling treatment Note the persistence of the lower 2θ peak

after poling to 1.5 kV/mm at room condition Sample thickness

is 1mm

2θ

(a) 0.2 kV/mm

(b) 0.7 kV/mm

(c) 1.5 kV/mm

(b) High-resolution synchrotron RSMs revealed that this surface layer consisted of

highly distorted R phase owing to the soft elastic constants of R The distorted R

phases are microscopic in size, of a wide range of lattice parameters and are

stressed in intense compression in the plane of the polished surface

(c) Under the PLM, the surface layer may exhibit parallel domain patterns when the

polishing was performed in a given direction but was relatively featureless

except regions of slight birefringence when the polishing direction was

uncoordinated

(d) This polishing-induced surface layer was fairly resistant to annealing but could

be largely eliminated by poling to 0.7-1.5 kV/mm at room temperature After

annealing, it became resistant to poling such that the lower 2θ peak persisted

even after poling at 1.5 kV/mm

(e) The present work showed that the diffractions arose from polished surface

pertained to surface phase while those from the fractured surface represented the

bulk phase The fractured surfaces are relatively strain-free surface which are

suitable for x-ray diffraction study of bulk structure in relaxor ferroelectric

single crystals